Motorcycle Repair: cb400f turn signal issue, turn signal switch, dual filament

Question
My left turn works fine - quick blinking and lights turn off entirely between
blinks. Right signals start off blinking but very slowly. The front signal doesn't
go out all the way between blinks and after a few cycles, they quit blinking
altogether and all three lights (front, rear and indicator) stay on.

Answer
DJ, Well, if the left side works fine, we can rule out low voltage (bad battery, weak voltage regulator, etc.) as a cause.
First, check to see what bulbs you have in your signal assemblies. Be sure that the ones on the left match the ones on the right. The fronts are usually 1157 bulbs, because they are dual filament, like the tail light bulb. The function on the fronts is that they are running lights when the headlight is on LOW beam and they go out when the HI beam is selected. The other filament is for the turn signal function.
It is very important that the bulb sockets are clean, the grounding inside the signal assy is correct and that the units, themselves are grounded to the frame through the green wires.
You could have some contact issues inside the turn signal switch, so split the housing and look inside to see if there are signs of corrosion. If you are careful, the switch can be disassembled for cleaning. It is a bit like watchmaking in there, so if you are not confident about doing that, leave it or replace the switch.
The headlight/starter switches on CB400Fs are known to fail internally, but that usually causes either the lights to fail or the starter not to work, as the switch enables/disables each function from the other, when you push the button. In the worst case, the fuse blows due to a short circuit inside the switch, when the switch case collapses and the hot contacts start getting all up against each other and the switch housing.

http://oldmanhonda.com/MC/WiringDiagrams/MCwiring.php#class has wiring diagrams, but I don't think they used the right one for the CB400F. The illustration for the 77 CB750F is an accurate illustration of the starter switch function, if that comes into play. The turn signal circuits are pretty much the same for all models, though.

Usually when you have this kind of problem, it is usually a bad ground, bad socket connections or wrong bulbs.

Bill Silver